The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Wellbore isolation during stimulation (for example by fracturing, acidizing, and acid fracturing) is performed by a variety of methods within the oilfield industry. One of the approaches involves the use of ball sealers, which are meant to seal the perforations and prevent fluid in the wellbore from flowing through the perforations into the formation.
Ball sealers are typically spheres designed to seal perforations that are capable of accepting fluid, and thus divert reservoir treatments to other portions of a target zone. Ball sealers are slightly larger than the perforations and are incorporated in the treatment fluid and pumped with it. They are carried to the perforations by the fluid flow, seat in the holes, and are held there by differential pressure. The effectiveness of this type of mechanical diversion requires keeping the balls in place and completely blocking the perforations, and depends on factors such as the differential pressure across the perforation, the geometry of the perforation, and physical characteristics of the ball sealers.
In some applications, the term perforation sealer or ball sealer refers to materials used in the oilfield in subterranean formations to seal-off perforations inside the casing. To differentiate between perforation sealers and particles, the perforation sealer has a cross sectional area that is equal to or larger than the single perforation in order to seat on and seal-off the perforation hole. Perforation sealers may be made of any material including degradable/dissolvable/soluble, elastic, deformable, hard, etc. substances and be of any geometrical shape such as spheres, cubes and cones.
Ball sealers are made in a variety of diameters, densities, and compositions, to adjust for different wellbore conditions and for perforation size. They may be either soluble or non-soluble. Soluble ball sealers are most commonly made of one soluble component, while non-soluble ball sealers often consist of a rigid core surrounded by a rubber (or other material) coating. The shortcoming of either ball sealer type lies in the relationship of the shape and composition of the ball sealer and the shape of the entry hole in the casing. Due to the nature of shooting perforations into casings, one obtains burrs and uneven surfaces that are difficult to seal with a smooth and/or spherical ball. In addition, an elongation of the entry hole may occur due to the casing curvature and the gun orientation when shooting perforations with a non-centralized perforating gun.
Generally, embodiments of this invention relate to fluids used in treating a subterranean formation. The majority of these stimulations are carried out as bullhead treatments utilizing perforation sealers and limited entry design as means of diverting fluid inside the wellbore to target zones. This technique gives satisfactory results in terms of diversion. However, wellbore pressure building is seen during subsequent “ball-off” of perforations as perforation sealers reach perforations, re-stimulation treatments lack the targeted pressure building effect from perforation sealers, and production response deteriorates as compared to early stimulation treatments. The root cause for this behavior is believed to be an ineffective sealing action of perforation sealers due to the following conditions, as well as a combination of these:
Corroded/eroded perforations
Scale forming around and on the perforation
Casing buckling/deformation:                Deformation of the metal as the area behind the liner is dissolved (formation) or degrades (cement)        Deformation of the metal due to pressure changes (during fracture treatments or during production)        
Altered perforation shape due to liner deformation
Casing collapse with and without cracks
The outcome is a perforation that is not as suitable a shape for seating the diverter ball as initially created when first made. Often, to counteract this behavior, the approach to resolve these problems is to drop more balls which, obviously, do not render a solution to this problem as proven by multiple treatments in the past.
A method that utilizes readily available components and is effective is needed. There is a need for improving the ability of ball sealers to close off perforations completely. A desirable method includes pumping suitable particles and fibers to plug the small flow paths that may otherwise remain in the perforations around the seated ball sealers.